I got the Power Wheels bug this year and am now up to 3 modified vehicles, all Craigslist specials. Nothing stays stock for long. My Kidnergartner and I made a trip out to the local offroad park on a very quiet afternoon and tried, unsuccessfully, to break one of the Quad Damage gearboxes. But we had fun trying!

And some Cul-de-sac racing and donuts with mostly-slick hybrid rubber/plastic tires:

Still trying to decide whether to invest more time in doing a full-suspension conversion ("build it") or just buy an actual ATV ("buy it"). Given my history, it's more likely to get built, though I may do it on the Jeep Hurricane instead. The F150 chassis is horrible.

That's awesome! It says 4x 775's are the front wheels powered too. And are those stock tires in the first vid? I wish more people would comment on cool things they see and learn from on this site, when you take the time to share the work you've put into something acknowledgement is also a reward. ; )

No, the front wheels aren't driven. This is a modified gearbox where two RS775 motors drive the 2nd gear in each gearbox directly. This changes the gearing ratio (by about 2.7:1 vs. driving 1st gear), but the motors in parallel, combined with higher torque motors and the ability to run various pinion sizes, means you can get some serious speed without dealing with steel 1st gears (and with relatively pedestrian motors, like the ones I'm using here). And you get the power dissipation area of two RS775 motors, meaning that it's going to take some serious work to overheat them.

I think this gives you about as much power as you can possibly run through 3rd and final gears on the gearbox and my biggest cost at this point is time (to make the aluminum plates), and the motor pinions. Honestly, the plastic gears have held up better than I expected.

The Mustang in the 2nd video is getting a 16 tooth version of this mod (the truck is running 11tooth pinions)-- should do 16-17mph @ 36V. I'm still working on rewiring and trying to figure out how to make the steering more robust (the steering wheel spline has gotten sloppy) before we get too crazy. It has "slip strip v2" on it. And it really should have a PWM controller, at 36V but I'll probably have one or both kids try it without it first. If I break a gear... well, I've got spares, and I'm planning on using relays at some point too, so I can handle smoking one of the stock switches (@ 36V it's pulling about the same current as the stock setups do at 18V).

As for tires -- in the offroad video the fronts had some road bike tires screwed onto them for pavement traction and some HDPE slip-strips; they're basically the rear tires from the other videos. The rears had 1/2" thick closed-cell foam (about 3-4" wide) covered by a stretched-flat mountain bike tire, screwed into the stock plastic tires. This gives it a bit of padding and seemed to work pretty well to soak up at least the small imperfections.